The present invention relates to a static reactive-power compensating apparatus for improving the stability of a power system by performing reactive-power control quickly in the event of a sudden change in voltage of the power system.
Conventionally, an arrangement as shown in FIG. 1 has been proposed as a reactive power compensating apparatus (hereinafter referred to as an SVC) for power system stabilization. In the drawing, reference numeral 1 designates a control quantity detecting circuit constituted by a rectifier circuit 1a, used for detecting a bus-bar voltage, and a reference voltage setting circuit 1b. Reference numeral 2 designates a thyristor control circuit, 3 a reactor, 4 a capacitor, and 5 thyristors.
FIG. 2 is a diagram showing the FIG. 1 circuit arrangement in block diagram form. When the voltage of the bus-bar to which the apparatus is coupled changes as shown in FIG. 3, reactive power, in an amount determined depending on the time constant Ty of the rectifier circuit 1a, the susceptance B.sub.L of the reactor 3, the susceptance B.sub.C of the capacitor 4, the control system gain K, and the dead time delay T.sub.D, is supplied to suppress the voltage change. Even a considerably quick voltage change can be responded to without delay in this arrangement since phase control of the thyristors is employed.
However, due to the use of a fixed value V.sub.C as a reference as shown in FIG. 2, the conventional SVC responds also to a voltage changes of a long period which can be remedied even by low-speed reactive-power control, that is, manual operation. Further, when a voltage change which has a short period is superposed on a voltage change having a considerably larger amplitude, the output of the SVC will reach its rated value during the suppression of the low-frequency voltage change, and hence a dead time may occur in which high-speed control to suppress the second transient cannot be carried out (hatched portions in FIG. 4).
It is a primary object of the present invention to eliminate the above-mentioned defects in the conventional apparatus.